The breaking up of masonry, particularly that constituted by reinforced concrete, causes problems due to the amount of noise and time involved. Whereas in the case of unreinforced masonry the demolition or knocking down and possibly also blasting can be carried out with different suitable aids, the possibilities are drastically reduced in the case of reinforced concrete masonry. Thus, all possible means are used for attempting to break up such structures, such as drills, hammers, tongs, jaws and other destruction means. However, this involves a great deal of noise and only has limited effectiveness.
As is now realized, concrete buildings were incorrectly designed as structures to last for centuries. Particularly in conjunction with their reinforcement, namely a stress-absorbing steel latticed girder construction, problems of durability (life) occur and consequently there are problems in breaking up and clearing such structures. The earliest experience obtained in this connection involved the clearing of the bunkers which had become useless after World War II. These tough structures could only be removed at the time manually using pneumatic hammers and blowpipes and the same still applies with the exception of a certain amount of manual work which is now carried out by machines. Reinforced concrete is difficult to demolish, whilst involving much noise and taking much time.
Concrete tongs or jaws would appear to be the most advantageous solution and they cut or crack the concrete. These hydraulically driven tongs are fast and quiet compared with the pneumatic hammers previously used. However, the equipment expenditure is considerable. Concrete tongs only operate in an optimum manner in a relatively narrow working field, so that for different wall thicknesses tongs with different "bite thicknesses" are used.
It is known that concrete masonry is relatively pressure-sensitive and under pressure a concrete layer tends to crack. This is utilized by the concrete tongs, which exert a wedge action on the concrete surface. However, extremely high breaking or crushing forces have to be expended. They represent 40 to 120 tonnes and over and this force must act on the forked levers, in order to be able to in this way break the concrete.
Thus, tongs are generally made which have a shape not dissimilar to lobster claws and said tongs normally have two tips per tong part. Such tongs press on the concrete with the indicated force in punctiform manner at several points (as in the case of a bit), and the points or tips form cracks through the concrete. This makes it possible to portionwise break away masonry. As soon as the cracking action has detached a concrete fragment from the bond, it still hangs on the reinforcement passing through the masonry and which must be separated in a further operating step. Either iron shears are used for this purpose, or blowpipes have to be used in order to separate the steel strands.
Although this appears to be very plausible, it is problematical to carry out. Firstly enormous forces are required to make such powerful bites. For a biting or breaking force of 50 tonnes between the tong pairs, it is necessary to have operating pressures up to 320 bar, which means that a pressure converter must be used, because almost 80% of shovels or excavators (i.e. the construction machine guiding and operating the breaking tongs) have a normal operating pressure of only 150 to 200 bar. Everything must be mobile and in particular the bite frequency must not be too small. To operate in a cost effective manner a large number of biting processes is necessary, together with a long life of the hydraulics and tong components. This is in turn a problem with regards to the machines and materials. Hydraulic cylinders for such high operating pressures are fault-prone and demolition work exposes them to a severe environment. The tongs can generally only meet these requirements, because they are relatively blunt and rough. However, the blunter the tong tips, the higher must be the tong force in order to achieve a satisfactory breaking action.